Low-sensitivity active-RC filters using impedance tapering of symmetrical bridged-T and twin-T networks

In this paper we introduce a new design procedure for low-sensitivity filter sections, which have a symmetrical passive-RC network in the operational amplifier (opamp) feedback loop. In the design procedure we apply impedance scaling to the symmetrical bridged-T and twin-T networks; they become "potentially symmetrical" (G.S. Moschytz, Linear integrated networks: fundamentals, V. Nostrand Reinhold, 1974). The design of low-sensitivity all-pole active-RC filters, which have an RC-ladder network in the opamp feedback loop, has already been published (G.S. Moschytz, IEEE Trans. on CAS-11, vol. CAS-46, no. 8, pp. 1009-1026, 1999). There, successive I-sections of the ladder structure are impedance scaled upwards, from the driving source to the positive opamp input; we refer to it as "impedance tapering". In both cases we reduce the filter´s magnitude sensitivity to variations of the passive components of a circuit. The new design concept is demonstrated by designing two very popular and often used filter sections: a band-pass realized by the Deliyannis SAB, and a band-rejection filter with a twin-T. The sensitivity analysis is examined analytically and double-checked using PSpice Monte Carlo runs.